A material is classed as nano when one of its dimensions exists in the nanometer range, i.e. is less than 100 nm.
Various methods exist to create nano sized materials. Zinc oxide (ZnO) is a particularly preferred nano sized material because it has various applications, including ultra-violet light absorbers in plastics, coating and paints. Zinc oxide is particularly useful as an activator in rubber vulcanization. In the case of zinc oxide, there are two main types of manufacture methods:    1. A gas stream process which uses a plasma stream of hot starting materials and regulates the particle size by temperature and flow rates, and    2. A sol-gel system which uses precipitation of the zinc oxide, out of a suitable solvent system in which the starting materials are dissolved.
In the method 2 above, it is often a necessity to use a suitable chemical agent to prevent the growth of crystals formed during such a reaction to limit the particle size that is achieved. This is done by introducing this chemical agent at a suitable time to allow interference with the crystal growth process. This is called capping, and many different chemical agents have been used.
In the specific case of zinc oxide by this sol process, a suitable zinc salt (like zinc acetate) is reacted with sodium hydroxide in a polar solvent mixture of water and some other alcohols. The choice of solvent is often determined by what is safer and cheaper to use.
In rubber vulcanization the use of zinc oxide is essential as it plays an important role in activating the vulcanization reaction by its role as an intermediate in the reactions of the accelerators, sulfur and the rubber. The amount of zinc oxide is essentially of the range 2 to 5 parts per hundred of rubber by mass. Considering current environmental considerations, it has become desirous to limit the zinc oxide amounts as it is potentially a toxic material in aquatic systems. While other metal oxides exists which can replace the role of zinc oxide, they are considerably more toxic to the environment.
The drive to use zinc oxide was dominated by the knowledge that the vulcanization reactions in the rubber material are related to the surface area of the zinc oxide—hence a nano zinc oxide should have a far greater available surface area per mass and this should favourably affect the vulcanization. However, this was not the case because pure nano zinc oxide had very small effects on the vulcanization which rendered the use of the material unsuitable, considering the cost.
When the reaction mechanism is considered for rubber vulcanization, the main intermediate on the zinc oxide surface is a zinc-accelerator intermediate which facilitates the vulcanization reaction. One of the earliest accelerators used in rubber vulcanization is 2-mercaptobenzothiazole (MBT) which accelerates the reaction between sulfur and diene rubbers. In particular, MBT accelerates the reaction of sulfur with rubber. ZnO in turn catalyses this reaction.
When nano zinc oxide is formed by method 2 (above) the capping agent is predominantly a sulfur based material as a result of the good bond between zinc and sulfur. The choice of capping agent is however limited by the polarity of such a material in that such material must be soluble in the (polar) solvent used. To date this has not been perceived as a problem in the art as there are many thiols available and most are soluble in polar solvents.
However, to date there has been no teaching in the art to cap the ZnO with non-polar materials which can be advantageously used in vulcanized rubber formulations. These materials have no or limited solubility in aqueous or alcoholic solvents.